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NEW YORK (CNN) -- There's no doubt that the recent immigration rallies attract very big numbers. But if you're trying to measure the political impact of such protests, history teaches us that...size isn't everything.

Sometimes it's what happens to protestors that galvanizes public opinion. As in 1932, when a Washington protest by World War I veterans for bonus pay was broken up by the Army led by Douglas MacArthur. That gave a big black eye to the Hoover administration.

When fire hoses were turned on civil rights demonstrators on the Selma-to-Montgomery March in 1965, it crystallized public opinion and led to President Johnson's famous promise to pass a voting rights bill. It was the culmination of a fight that gained major impetus from another demonstration --the 1963 March on Washington, where Martin Luther King delivered his classic "I have a dream" speech.

That march was focused on a specific objective. By contrast, the "Million Man March" in October, 1995, drew far more participants than Dr. King's march, but it was not aimed on any specific piece of legislation or policy. That, and its sponsorship by the controversial Minister Louis Farrakhan, helped dissipate its impact.

And even the biggest demonstrations haven't had much impact when the battle lines are already clearly drawn -- both sides of the abortion debate have turned out large numbers in Washington over the years...to no apparent impact. But it's also true that small crowds don't necessarily measure popular sentiment; the protests marking the third anniversary of the invasion of Iraq drew relatively tiny crowds.

But the news from Iraq since then has led to a steady drop in support for the president's Iraq policies. That drop can't be measured by people in the streets, but by the increasingly skeptical nature of the "national conversation," including comments by retired generals like Paul Eaton and Gregory Newbold, who say either that the war was a mistake from the beginning, or that the execution of a post-war plan has been a failure.

As for these demonstrations: do they show widespread support for creating a path to citizenship? If that's the case, they could be compared to the enormous -- sometimes violent -- protests in France opposing a new law giving employers the right to fire new employees. French President Chirac has announced that plan is dead.

Or might they trigger a backlash among Americans...most of whom, the polls say, want more emphasis on enforcement and border security? Along with a sea of American flags, some of the demonstrators have carried Mexican flags, portraits of slain Cuban revolutionary Che Guevara, and the (very) occasional racial epithet aimed at whites. In that case, they might be compared to the more radical protests of the Vietnam Era, their Viet Cong flags and calls to "Smash the state" triggered a decidedly unsympathetic response.

What we can know, with some certainty, is that numbers alone don't begin to tell us what impact these events will have.